PS5 Pro is upscaling from native 1440P which is about 26% more raster than 1280P and yet it runs 20% faster as the target performance is 40 fps vs 48 fps.
Even if we assume that the cost of FSR3 is similar to PSSR (which of course is not true as proven in multiple PS5 Pro games)
There's about 26% (from the res difference) multiplied by 20% (from the performance difference) which sums up to over 50% performance disparity between PS5 and PS5 Pro.
Considering how PS5 Pro is supposed to raster up to 45% faster (which in most cases about 20~30%) It is safe to assume that
RT low on base PS5 is a bigger burden than RT high on PS5 Pro with it's 2~3 times RT capability.
And you're expecting not having FSR3 will mitigate that 50% or even exceeds that the game will run faster on base PS5?
Well, we'll find out once the game is available